(1) Field of the Invention
The present invention relates generally to a structure of a heat pipe, and more particularly to the structure of the heat pipe which can be provided with small-sized, light-weighted, heat receiving and heat radiating apparatuses for the heat pipe and can achieve a very long heat pipe of continuous capillary dimension having very narrow inner and outer diameters which could not conventionally be manufactured.
(2) Description of the Background Art
Recently manufactured metallic capillary heat pipes tend to have a performance changed remarkably according to a mounting posture thereof. Particularly, it is almost impossible to operate the capillary heat pipe mounted under a top heat situation, i.e., under a state where a water level of a heat receiving portion of the heat pipe is higher than that of the heat radiating portion.
Since, in operation, a vapor stream of a working liquid which moves from a vapor portion to a condensating portion at high speeds and a stream of condensated liquid which circulates from the condensating portion to the vaporizing portion are mutually in opposite directions, their mutual interference make the cause difficulty in utilizing a smaller or fine heat pipe dimension. Therefore, there is a limit of manufacturing a fine capillary heat pipe having an outer diameter of approximately 3 mm and a length of approximately 400 mm. As a matter of fact, in the capillary heat pipe generally referred to as a micro-heat pipe, the length of merely several to 10 mm is the limit of manufacturing the heat pipe.
It is impossible to bend the loop portion of a loop-type heat pipe and, a degree of freedom in use is problematically small.
U.S. Pat. No. 4,921,041 issued on May 1, 1990 and Japanese Patent Application First publication No. Showa 63-31849 published on Dec. 27, 1988 exemplify previously proposed heat pipe structures which solve the above-described problems.
One of typical previously proposed capillary heat pipe structures (refer to FIG. 2) includes: a continuous elongate tube (2) of continuous capillary dimension having both ends thereof air-tightly connected to each other to form a continous capillary loop-type flow passage; a heat carrying fluid within the elongate tube in a predetermined amount sufficient to allow flow to the fluid through the loop flow passage in a closed state defined by the elongate tube; at least one heat receiving portion (2-H) located on a second part of the elongate tube for heating the fluid therein; at least one heat radiating portion (2-C) located on a second part of the elongate tube for cooling the fluid therein; and flow control means (3) located within the loop-type flow passage for limiting flow of the heat carrying fluid to a single direction in the flow passage. Especially, a bi-phase condensative working liquid (4) is filled in the container as a heat carrying fluid. It is noted that an inner diameter of the capillary tube is smaller than a maximum of the inner diameter which could circulate or travel with the working fluid always closed in the tube due to the presence of a surface tension of the tube.
The flow control means is constituted by at least one check valve (3).
In the structure of the loop-type heat pipe described above, external heating means (H) is provided to heat the heat receiving portion (2-H) while the heat radiating means (C) is externally provided to cool the heat radiating portion (2-C). At this time, the check valve serves to separate the loop-type container into a plurality of pressure chambers in which a nucleate boiling (5) generated within the heat receiving portion causes a vibrative pressure difference and an inspiring action to be generated between the plurlity of pressure chambers formed by means of the check valve(s). The nucleate boiling within the heat receiving portion serves to propagate a pressure wave in the fluid, the pressure wave causing a valve body to be vibrated. Mutual actions between the vibration of the check valve body and inspiring action integrally generate a strong circulation propelling force on the working fluid.
In the way described above, the bi-phase working fluid in itself circulates in the predetermined direction within the loop. The nucleate boiling is not continuous. Thus, the circulating working fluid (4) circulates with its vapor bubbles (5) and working fluid (4) (closed liquid droplets) alternatingly arranged. Hence, heat transportation occurs due to a latent heat by heat transfer of the working fluid and sensible heat of the vapor bubbles (5).
The heat transportation due to the circulation stream of the working fluid makes possible an excellent heat transportation capability, irrespective of mounting posture of the heat pipe. In addition, since the heat pipe has a capillary dimension, the small-sized and light-weighted heat pipe can be achieved. Since it is possible to use the heat pipe in the free bending form, the degree of freedom of using the heat pipe can remarkably be enlarged.
However, the previously proposed heat pipe structure has yet various problems to be solved although the excellent performance is exhibited irrespective of the mounting posture in use and the heat pipe (refer to FIG. 2) can freely be flexed.
The problems yet to be solved are to promote further miniaturization of the diameter of the heat pipe in a micrometer range and reduction in weight of the heat transporting apparatuses and heat receiving and heat radiating apparatuses to meet demands by the technological field of the heat pipe.
In more detail, the problems yet to be solved are listed below:
a) If a thinner diameter of the heat pipe container is put into practice with the inner diameter of about 1.2 mm as a boundary, a failure rate of product (inverse of yield of the product) is abruptly increased and reliability is remarkably reduced. In a case where the check-valve equipped loop-type heat pipe is manufactured, the check valve has a very small dimension so that a quality control of the heat pipe during its manufacture cannot be assured.
A plurality of junctures are required for manufacturing the actual loop-type heat pipe disclosed in U.S. Pat. No. 4,921,041. As shown in FIG. 3, the required junctures are such as junctures (3-1, 3-2, 3-3) for mounting the check valve(s), junctures (8) for the connection of each heat pipe portion to form the loop, junctures (9) for injection of the working fluid into the inner portion of the capillary tube (2), and gas exhaust junctures (10) for the capillary tube. Welding operations for the respective junctures are carried out during manufacture. For example, the junctures (3-1, 3-2, 3-3, and 8) need to be welded at their two parts, the junctures (9, 10) need to be welded at their four parts. Therefore, an abrupt difficulty in the welding operations occurs in heat pipes having an outer diameter less than 1.6 mm and inner diameter less than 1.2 mm. Consequently, the reliability of the product becomes reduced.
b) It is difficult to guarantee a long term reliability for a large thermal input at high temperatures even if a ruby-made ball is used as a valve body of each check valve. During a reliability test of a heat radiator requiring impulsively the thermal input of 5 KW at 300.degree. C., such an accident as the destruction of the ruby-made ball has happened. Then, the ruby-made ball was replaced with a tungsten carbide ball and the reliability test was performed. Since the relative weight was as large as 13, the operation at the time of low thermal input was worsened. In addition, due to too much relative weight, a floating operation became difficult and the impulse of opening and closing the valve was generated. This indicated that the long term reliability was not guaranteed.
c) A limit of selection of a metallic material for the capillary container is present in order to guarantee the long term reliability of the check valve.
The reliability test for the check valve equipped loop-type heat pipe indicated that, according to a metallic material used for the internal surface of the capillary tube, an intergrunular corrosion occurred in metallic crystallines of the inner surface of the metallic capillary tube and multiple quantities of metallic powders were freed and deposited on each check valve, whereby heat transport operation was prevented
d) If a floating type of check valve is used as disclosed in the U.S. Pat. No. 4,921,041 in order to elongate the life guarantee period, a reaction force, due to leakage loss in the check valves, is so weak that a water level difference between the heat receiving and heat radiating portions is limited to about 1000 mm by which the heat pipe is used in the top heat mode.